An ear is comprised of three basic parts an outer ear, a middle ear and an inner ear. Sound travels from the outer ear to an eardrum at the interface of the outer ear and then to the middle ear in the form of vibrating air waves. Then, it is transmitted to the oval window of the cochlea through the vibrations of the auditory ossicles. With the vibration of fluid in the cochlea, corti organ produces an electrical signal.
The vibration of the tectorial membrane leads to the generation of a receptor potential in the hair cells, which is transmitted to the spiral ganglion through dendrites. The signal from the spiral ganglion stimulates neurons of the auditory nerves and is finally transmitted to the cerebrum. Most hearing loss is due to poor function of the hair cell in the cochlea. In this case, the hearing loss can be alleviated by providing an artificial electrical stimulation, in replacement of the firing of hair cells. Cochlear implants have been developed based on the fact.
A cochlear implant is an electronic device surgically implanted into the body that provides a sense of sound to a person who is suffered from severely low hearing. The cochlear implant directly stimulates an auditory nerves inside the cochlea with electrical impulses. Cochlear implants provide good hearing ability to those who have severe hearing impairment in both ears and are not benefited by hearing aids. The cochlear implant is evaluated as the most successful nerve aiding device developed thus far.
Since approved by the United States Food and Drug Administration (FDA) in 1984, cochlear implants have been widespread. Recently, a multi-channel cochlear implant has been proposed. The multi-channel cochlear implant requires multiple electrodes in order to produce individual frequency signals, each of which is mapped to a particular location in the cochlea. Electrode signals that match the frequency requirements are transmitted individually to the dendrites below impaired hair cells.
FIG. 1 is a schematic view of the internal receiving unit (10) of the conventional cochlear implant having multiple electrodes. It comprises an active electrode (10a) and a reference electrode (10b), wherein end of the active electrode (A) is spirally shaped for convenient insertion into the cochlea. FIG. 2 shows an exploded view of the end of the active electrode (A). At the end of the active electrode (10a), multiple electrodes (15) are installed, each of which providing an electrical stimulation of a particular frequency to the specific area of the cochlea.
The cochlear implant comprising the multiple electrodes is as long as approximately 35 mm and may impair the hair cells while being inserted into the cochlea. Further, the spiral end of the electrode may cause damage to various organs such as the hair cells, dendrites or spiral ganglion when it touches the inner wall of the cochlea and may lead to a permanent hearing loss.
Patients with multi-channel cochlear implants show approximately 80% of sound recognition in the noiseless, silent environment. However, the level of recognition decreases in noisy place where many people are talking. Also, they have difficulty in enjoying music and, particularly, show significantly poor tonal sensibility.
Besides, the conventional multi-channel cochlear implants are suffered from various disadvantages such as expensive cost, very complicated surgery and risk of ins anitariness.